Electronic tag detection systems include article surveillance systems and asset tracking systems, and are known in the art wherein tag detection is carried out by transmitting an electromagnetic field into a detection zone. In these systems, determining the presence of the articles under surveillance is accomplished by sensing perturbations to the transmitted electromagnetic field. Perturbations to the transmitted electromagnetic field are generated by electronically detectable tags attached to or incorporated into the articles. These tags carry or are formed from magnetic markers, materials, or circuits, which create the perturbations, and can be simple tags or complex tags, and may carry one or more bits of data.
One type of electronic tag detection system utilizes a single magnetic field and simple tags. Simple tags employ no multi-tag algorithms. For example, such tags are not addressable and cannot vary transmission times. In single field systems utilizing simple tags, a single antenna is placed on one side of an opening. Ideally, the antenna transmits a magnetic field of a particular minimum intensity, which occupies the entire opening. A receiving antenna is placed opposite the transmitting antenna on the opposing side of the opening. The area between the transmitting antenna and the receiving antenna is called the detection zone. Alternatively, a transceiver system can be employed wherein the transmitting antenna and the receiving antenna are located within the same physical housing on one side of the opening. In either case, the resulting magnetic field from the transmitting antenna is effective for triggering a response from simple tags passing through the detection zone. For example, the magnetic field has a minimum intensity required to trigger a response from a tag passing through the magnetic field. When a tag attached or incorporated into an article passing through the detection zone is exposed to a magnetic field having a particular frequency and magnetic field intensity, the tag causes perturbations in the magnetic field. These perturbations are typically in the form of harmonics of the fundamental frequency of the magnetic field, which can be detected by the receiving antenna.
One significant problem with single field electronic tag detection systems using simple tags is the system's inability to detect more than one tag within the detection zone at one time. For example, when two tags are present in the detection zone at the same time, each tag simultaneously generates perturbation energy that is received by the receiving antenna. Although the antenna receives the perturbation energy in the form of harmonics of the fundamental frequency of the magnetic field from both of the tags concurrently, the tags interfere with one another. Such interference between the tags results in the system receiving corrupted data. Consequently, the system is unable to determine whether a tag has passed through the detection zone.
Another type of electronic tag detection system utilizes two magnetic fields and simple tags. In this system, a transceiver is located on one side of an opening, and a second transceiver is located on the opposite side of the opening. The area between the two transceivers is referred to as the detection zone. The two transceivers are tuned so that the transmitting antenna of each transceiver transmits a magnetic field extending to the middle of the detection zone. The two magnetic fields do not overlap or under-lap each other. Moreover, no portion of the detection zone is left unoccupied by a magnetic field. Thus, if two tags pass through the detection zone at one time, use of a two magnetic field system increases the likelihood that one tag will pass through the left magnetic field and the second tag will pass through the right magnetic field. Consequently, the likelihood of receiving corrupted data due to more than one tag passing through a single magnetic field at one time is decreased.
There are however significant problems with two field electronic tag detection systems. One problem is that two field systems cannot detect more than two tags within the detection zone at one time. If three or more tags pass through the detection zone, then at least two tags will pass through a single magnetic field. As was the case with single field electronic tag detection systems, two tags within a single magnetic field interfere with one another. Consequently, the system receives corrupted data. Another problem is that two field systems rely on the contingency that if two tags pass through the detection zone simultaneously, each tag will pass through a different magnetic field. For example, such systems will receive valid data only if one tag passes through the left magnetic field while the second tag passes through the right magnetic field. However, as the size of the detection zone increases, the size of the magnetic fields necessary to occupy the detection zone increase as well. Consequently, the likelihood of two tags passing through the same magnetic field simultaneously increases with the size of the detection zone. Thus, the benefits of a two field electronic article surveillance system are not realized as the detection zone becomes larger.
In the known art, U.S. Pat. No. 5,049,857 (the '857 patent) discloses a magnetic article surveillance system and is incorporated herein by reference. The '857 patent switches between a transmit/receive mode and a transceiver mode to better distinguish electronic tags from items that may cause false alarms. The surveillance system utilizes a transmitter for transmitting magnetic energy into a detection zone and a receiver for receiving magnetic energy from the detection zone, and alternately a transceiver performing the same functions as the transmitter and receiver. The transmit/receive mode and the transceiver modes each provide different detection characteristics for determining false alarms, which the '857 patent takes advantage of by incorporating both for detection. Electronically detectable tags passing through the detection zone are detected by the system even in the presence of interfering materials. The '857 does not, however, teach how to differentiate multiple valid tags in the detection zone. It should be appreciated that magnetic article surveillance systems and asset tracking systems utilizing transmitters and receivers and/or transceivers for receiving and transmitting magnetic energy into a detection zone for the detection of electronically detectable tags are known in the art.
As such, electronic tag detection systems using simple tags suffer from the deficiency of not being able to reliably detect more than one tag within the detection zone at one time. As a result, there has arisen a need for an electronic article surveillance system capable of detecting multiple tags in a detection zone without using complex multi-tag algorithms.